1. Field of Invention
This invention relates to actuator mechanisms and is particularly directed to electrically controlled means for selectably causing a mechanical motion having substantiaL tolerance and for assuring maintenance of that motion for a desired interval.
2. Prior Art
Electrically driven actuating mechanisms have long been known and are widely used for many purposes. Thus, for example, solenoids are widely used to control mechanical movement in a great many devices. However, most of these solenoid-controlled devices require abrupt mechanical movement from a first position to a second position and the locations of these positions must be quite precisely fixed. Also, solenoid devices are relatively noisy. However, for many purposes, smoothness of operation is preferred to speed and silence of movement is highly desirable. Furthermore, it may be desirable to allow considerable tolerance in the location of the first or second positions. One example of these problems is found in providing actuating means for model railroad switches. It is common practice, in model railroading, to construct a railroad layout incorporating components, such as track switches, produced by a plurality of manufacturers. Unfortunately, different manufacturers have developed track switches and track gauges which each have respective amounts of travel. Consequently, a switch actuator, which is intended for use with a plurality of such track switches, must have sufficient tolerance to accommodate these respective amounts of travel in order to properly close the track switch to control the direction of travel of the train and to prevent derailment. In the past, motorized actuation has been required to obtain such slow, smooth movements. Recently, shape memory wire has been used to produce such movement. Shape memory wire is an alloy of nickel and titanium, generally provided in the range of 4-10 mils diameter, having the property of rearranging its molecular and crystalline structure at a specific threshold temperature and returning to the original state upon cooling. The result of this rearrangement is a physical shortening of 4-5% of the length of the wire, although the volume remains constant. Moreover, the resulting motion is strong, smooth and silent. The most common method of obtaining the internal heating necessary is by the application of electric current directly to the wire. Unfortunately, overheating or interfering with the constricting motion has been found to cause stresses which damage or destroy the ability of the alloy to function in this manner. Thus, in the case of the model railroad track switch actuators, if an actuator, which was designed for a track switch having a large range of travel, is installed to control a track switch having a small range of travel, closing of the track switch will interfere with the constricting movement of the wire and, hence, will damage the actuator. On the other hand, an actuator which is designed for a track switch having a small range of travel will fail to properly close a track switch having a large range of travel and, hence, will either fail to control the direction of travel of the train or will cause derailment. Accordingly, none of the prior art actuating mechanisms have been entirely satisfactory.